Heat stimulation of fractured wells



Jan. 4, 1966 R. E. GILCHRIST HEAT STIMULATION OF FRAGTURED WELLS FiledDec. 17, 1962 INVENTOR.

L/STRATUM on. STRATUM SUBSTRATUM R E. GILCHRIST A TTORNEVS United StatesPatent HEAT STIMULATION 0F FRACTURED WELLS Ralph E. Gilchrist, Bartlesville, Okla, 'assignor to Phillips Petroleum Company, a corporation ofDelaware Filed Dec. 17, 1962, Ser. No. 245,031 6 Claims. 1 (Cl. 166-11)This invention relates to a process for the production of hydrocarbonsfrom an oil bearing stratum by heat stimulation. i

The application of heat to an oil-bearing stratum around a well thereinis conventional practice. The principal difliculty in. heat stimulationof a well by this technique to produce more oil from the stratum is incausing the heat to penetrate deep enough into the wellto producesubstantial amounts of oil therefrom. This invention is concerned with amethod or technique for heating a stratum containing oil and,particularly, heavy viscous oil or semi-solid to solid hydrocarbonmaterial at normal stratum temperatures.

Accordingly an object of the invention is to provide an improved processfor the production of hydrocarbons from a fractured stratum around awell therein by heat stimulation thereof. Another object is to provide amethod of propping a fracture open so that heating to a substantialdepth in the stratum by means of a propping agent is feasible. Otherobjects of the invention will become apparent to one skilled in the artupon consideration of the accompanyingdisclosure. a

A broad aspect of the invention comprises filling a fracture around awell in an oil-bearing" stratum with meta1 pellets to form a continuouslayer of the pellets in the fracture which serves as a heat'conductingmedium for heating the stratum remote from the well by application ofheat to the pellets contiguous to the well. i The heating is effectedwithout in situ combustion or in the absence of free-oxygen, by means ofa suitable down-hole heater such as a gas-fired burner or an electricheater. The pellets may be composed of any metal or alloy of metals butpellets of aluminum or aluminum alloy, consisting essentially ofaluminum, are preferred because of their good heat conductance and theirrelatively low density which facilitates the injection thereof intofractures in the stratum to be produced. Magnesium pellets or pelletscomposed of an alloycontaining magnesium, such as Mg-Al, are alsosuitable but more expensive than those composed principally of aluminum.

The use of aluminum pellets in propping fractures in a stratum toincrease the production of oil has been practiced heretofore. However,in such practice the alumi num pellets or those formed from an aluminumalloy have not been injected into the fracture in suflicientconcentration to provide pellet to pellet conduction of heat thru thefracture deep into the stratum as is provided in the process of thisinvention. A description of conventional practice is found in thePetroleum Engineer, November 1960, at page B-40 in an article by J. E.Kastrop entitled, New Fracture Propping Process Uses Aluminum Pellets.In the practice discussed therein the well is fractured by any suitablemethod, such as injecting a fracturing fluid comprising a gas, a liquid,or a mixture of gas and liquid, and the pellets are injected into thefractures or tailed in in suspension or dispersion in a suitable carrierfluid which may comprise gelled water, diesel oil, kerosene, and similargelled heavy hydrocarbons. Waterin-oil emulsions are also suitable. Aconcentration of pellets in the carrier fluid of at least 5 pounds pergallon of fluid is required to provide a continuous layer of pellets inthe fracture. After the propping pellets have been tailed in to form acontinuous layer in the fracture, the pressure is reduced in theinjection well and the excess 3,227,211 Patented Jan. 4, 1966 pelletsare removed from the well in conventional manner.

The pellet size to be used is in the range of 6 to 16 mesh. The layer ofpellets in the fracture may be more than one pellet thick so as tofacilitate the heat conduction during the heating step. The pellets maybe of uniform diameter or of mixed diameters within the above range.

A more complete understanding of the invention may be had by referenceto the accompanying schematic drawing which is an elevation in partialsection of an arrangement of wells and fractures in a stratum for use inaccordance with the invention.

Referring to the drawing, a stratum 10 is penetrated by wells 12 and 14which are cased substantially to the upper level of the stratum bycasings 16 and 17, respectively. Tubing string 20 extends thru the wellhead 22 on casing 16 and carries a downhole heater 24. A similararrangement of tubing 26, well head 28 and heater 30 is provided in Well14. Stratum 10 is fractured by conventional method around each of wells12 and 14 at a selected level to produce fracture 32 which extendslaterally outwardly from each wall so as to intersect intermediate wells12 and 14' and provide communication therebetween. After fracturing,metal pellets 34 are tailed in by injecting a suspension of pellets in asuitable heavy or viscous liquid and at sufiicient concentration (atleast 5 pounds of aluminum pellets per gallon of carrier fluid) to forma continuous conductive layer of the pellets extending radiallyoutwardly from each well. After the continuous solid conductive layer ofpellets is positioned in the fracture, downhole heaters 24 and 30 arepositioned opposite or at the same level as the fractures and theheating step is effected with continuation thereof until heat has beenconducted into the stratum laterally in all directions at least severalfeet and preferably until the stratum has been heated substantiallyabove normal temperature as far out as the continuous layer of pelletsextends. The heating period may take several days or weeks, dependingupon the depth to which the pellets extend from the wells and the extentof the section of stratum heated. The heating of the stratum along thefractures by conduction of heat thru the pellets has the effect ofrendering the in-place hydrocarbon material more fluid or less viscousso that the resulting oil flows into the well, thereby increasingproduction from the stratum. Heating can be continued until heatpenetrates deep from the fracture. The economics of the process willdetermine the length of the heating period.

In thick strata, it is desirable to fracture at more than one level asat 36 in the drawing and again fill the fracture with a continuous layerof metal pellets 38 followed by heating the pellets by means of a heaterat the well bore and allowing the heat to be conducted deep into thestratum. The heating in the different fractures is preferably conductedsimultaneously, but it may be effected successively.

In one embodiment of the invention the stratum to be produced isfractured at different levels around one well or a plurality of wellsand heat stimulation is applied to the metal pellets in the fracturesuntil the stratum intermediate the fractures is heated and produced 'soas to be sufficiently permeable from removal of oil, semi-solid, orsolid hydrocarbon material to permit production thereof by in situcombustion. The stratum is ignited while hot from the heat stimulationstep by injecting air thru one of the fractures and withdrawingcombustion gas and produced oil thru the other fracture and a wellconnected therewith, thereby moving a combustion front vertically thruthe stratum. The combustion front may be moved thru the stratum eitherby counterflow of air, providing the stratum is allowed to cool belowignition temperature before injecting air thru the fracture remote fromthe ignition fracture. In direct flow, the air feeding the combustionzone is fed thru the fracture in which combustion is initiated. Incounterflow combustion, the stratum is ignited in one fracture byheating the adjacent stratum and injection of air thereto and, aftercombustion is initiated, the flow of air thru the ignition fracture isterminated and the air is injected thru the other fracture so that itreestablishes combustion at the original combustion zone and causes thecombustion front to move inversely or countercurrently to the flow ofair to the injection fracture. A hot flushing gas may be passed thru thefractures and/ or thru the stratum between fractures to increase oilflow.

The process of this invention may be effected around a single well, suchas well 12, without the use of offset wells, such as well 14. In thistype of operation the same fracturing and propping technique is utilizedand the heating is applied to the continuous conducting layer of pelletsin one or more fractures at different levels around the well to causethe hydrocarbon material in the stratum to become more fluid and flowmore readily into the well for recovery by conventional techniques suchas by pumping. Also, it is desirable to apply the heat stimulation stepin spaced apart fractures around the same well at different levels untilthe intervening stratum has been increased in permeability sufiicientlyto permit movement of a combustion front vertically thru the stratum andthereafter effecting this procedure so as to denude the stratum ofcarbonaceous material.

The process of the invention is applicable to virgin strata as well asto the recovery of hydrocarbons from strata which have been produced bynatural gas drive or by other means, such as fluid flooding and misciblefluid drive. The process is particularly applicable to strata containingheavy hydrocarbon material which renders other methods of productiondifficult or inoperable.

The process may be effected in wells in a 5-, 7-, or 9- spot pattern aswell as in parallel lines of wells. Again referring to the drawing, well12 may be considered a central well surrounded by a ring of wells 14.Well 12 may also be representative of one well in a line of wells whichis generally parallel with a second line of wells in which well 14 islocated. There may also be another line of wells parallel with and onthe opposite side of wells 12 from wells 14.

Certain modifications of the invention will become apparent to thoseskilled in the art and the illustrative details disclosed are not to beconstrued as imposing un- 0 necessary limitations on the invention.

I claim:

1. A process for heating a stratum penetrated by a pair of wells atspaced points and recovering oil therefrom which comprises the steps of(a) fracturing said stratum at a selected level around 4 each well sothat the fractures provide communication between said wells;

(b) injecting small metal pellets into the resulting fracture in acarrier fluid in a concentration of at least 5 pounds per gallon offluid;

(c) continuing the injection of pellets and fiuid until a continuousconductive layer of pellets extends from well to well;

(d) heating the pellets contiguous to both wells by means of downholeheaters so as to conduct heat from said heaters thru the layer ofpellets from well to well;

(e) continuing the heating step so as to further heat said pellets andthe adjacent stratum and cause oil to flow into said wells;

(f) recovering the oil thus produced;

(g) repeating steps (a) thru (f) at a second level in said stratum andcontinuing the heating until the stratum intermediate said levels issufficiently permeable to permit producing same by in situ combustion;

(h) thereafter, while said stratum is hot, igniting same along thefractures at one level by injecting air thereto thru said wells;

(i) continuing the injection of air so as to move a combustion zone thruthe stratum from one fracture level to the other; and

(j) recovering the produced hydrocarbons thru at least one of saidwells.

2. The process of claim 1 wherein prior to said in situ combustion stepa hot non-oxidizing flushing gas is passed thru the fractures from onewell to the other to flush oil from said stratum to increase oilrecovery.

3. The process of claim 1 wherein said metal pellets consist essentiallyof aluminum.

4. The process of claim 1 wherein each well is substantially free ofsaid pellets and heat is transmitted from said heaters to the pellets insaid fractures principally by radiation and convection.

5. The process of claim 1 wherein said pellets are of a size in therange of 6 to 16 mesh.

6. The process of claim 5 wherein said pellets consist essentially ofaluminum.

References Cited by the Examiner UNITED STATES PATENTS 2,742,967 4/1956Carpenter 166-39 2,818,118 12/1957 Dixon 166-11 2,946,382 7/1960 Tek etal 166-421 X 3,010,513 11/1961 Gerner 166-11 3,018,827 1/1962 Hendersonet al 166-39 3,105,545 10/1963 Prats et a1 166-11 X CHARLES E. OCONNELL,Primary Examiner.

EENJAMIN HERSH, Examiner.

1. A PROCESS FOR HEATING A STRATUM PENETRATED BY A PAIR OF WELLS ATSPACED POINTS AND RECOVERING OIL THEREFROM WHICH COMPRISES THE STEPS OF(A) FRACTURING SAID STRATUM AT A SELECTED LEVEL AROUND EACH WELL SO THATTHE FRACTURES PROVIDE COMMUNICATION BETWEEN SAID WELLS; (B) INJECTINGSMALL METAL PELLETS INTO THE RESULTING FRACTURE IN A CARRIER FLUID IN ACONCENTRATION OF AT LEAST 5 POUNDS PER GALLON OF FLUID; (C) CONTINUINGTHE INJECTION OF PELLETS AND FLUID UNTIL A CONTINUOUS CONDUCTIVE LAYEROF PELLETS EXTENDS FROM WELL TO WELL; (D) HEATING THE PELLETS CONTIGUOUSTO BOTH WELLS BY MEANS OF DOWNHOLE HEATERS SO AS TO CONDUCT HEAT FROMSAID HEATERS THRU THE LAYER OF PELLETS FROM WELL TO WELL; (E) CONTINUINGTHE HEATING STEP SO AS TO FURTHER HEAT SAID PELLETS AND THE ADJACENTSTRATUM AND CAUSE OIL TO FLOW INTO SAID WELLS;